The invention relates broadly to eye-implantable means for impeding secondary growth within an eye; more particularly, the invention also relates to such means for impeding or preventing cell growth over one or more optically used surfaces after an intraocular-lens has been implanted, as in the case of a posterior-chamber lens mounted in the lens capsule of an eye, following extracapsular cataract extraction.
In the United States, the use of lens implants in replacement of cataract extractions has been a growing practice since 1975, to the point that this year the vast majority of the million or so patients having a cataract extraction will receive an intraocular-lens implant. Such implants have almost totally replaced the use of thick eyeglasses or contact lenses as a means of correcting refraction and restoring close-to-normal vision.
Over this 16-year period, the techniques of removing a cataract have undergone great change. A large majority of the diseased tissue was removed using a technique known as "intra-capsular extraction". This technique requires making a relatively large incision in the eye, and removing the lens of the eye entirely. Intraocular lenses are then inserted to be supported by the iris, or in the anterior chamber.
This technique has its limitations, so that equipment and techniques have been developed that allow the lens capsule to be cut, a probe inserted, and energy applied to fragment the nuclear lens material. Through a technique of ultrasonic or pulsed laser delivery (phaco-emulsification), in conjunction with irrigation and aspiration, fragmented lens-cortical material is removed from the eye.
There are two methods of placing an artificial lens. Some physicians choose to place a lens anteriorly to the lens capsule. Others choose to insert it within the human capsule, a technique known as capsular fixation. The majority of lens implants today use the capsular-fixation technique.
After extra-capsular extraction, and intraocular-lens implantation, a large number of these lenses become opaque, by reason of epithelial-cell growth and protein-strand development over one or more optically used surfaces. In some cases this occurs immediately, and in other cases, it occurs many months after surgery. Ophthalmologists have developed techniques of scraping and cleaning the human-lens capsule in order to remove human-lens epithelial cells and thereby reduce the amount of opacification. However, because of the size and relative concentration of these cells at the capsular perimeter or equatorial zone, such techniques have not been completely successful, in that those cells which remain after a scraping or cleaning procedure are available to replicate and spread, and thus to form an opacifying epithelium layer of protein strands, over the optically used surface of the implanted lens. As a result, post-operative secondary procedures are required to clear the lens, and it is evident that these intra-operative techniques cannot be considered to be successful.
In addition to intra-operative cleaning procedures to remove these capsule cells, two alternative post-operative techniques are in use today, to clear the epithelial cells and protein strands from an opacified lens. The more common technique uses a YAG laser to direct a high-energy burst at the opacifying membrane. After repeated bursts, the membrane is ripped and a large enough area is cleared to return vision to the patient. The procedure may have to be repeated frequently on certain patients.
The alternative procedure involves manipulated insertion of a small needle or other instrument into the eye, under a local or general anesthesia. The physician observes the membrane and pulls the membrane away from the optic.
Both of these alternative procedures entail significant risks to the patient, not to mention the cost and inconvenience to the patient. Additionally to the re-growth of epithelium and protein strands, some implanted lenses opacify because they accumulate pigment or other deposits which reduce the optical properties of the lens.
Dr. Hoffer and his U.S. Pat. No. Re. 31,626 are illustrative of the fact that ophthalmologists have long recognized the opacification problem with intraocular-lens implants. His particular contribution to solving the problem was to form a sharp annular ridge or "dam" feature into the posterior surface of an intraocular lens, and this dam establishes peripheral contact with the posterior capsule, to prevent proliferating-cell advance from the equatorial zone to the optically used region of the posterior surface of the lens. Even so, Dr. Hoffer recognized that cell re-growth could ultimately be the cause of further opacification of the lens, because he provided an access space in the otherwise circumferential continuity of his ridge, so that a surgeon could manipulate a knife through the access space, to remove an opacifying accumulation from the posterior optical surface of the implanted lens. But the extent of professional debate on the Hoffer approach casts doubt on efficacy of the technique.